According to JohnB, Sunline got a bad batch of vents. Sorry to hear you've been bitten by the same problem. The vent frames are galvanized steel. I would not mess with trying to extend their life. Vents are a pain to remove and easy to bend as they are being pried up. IMHO you're far better to ditch the defective ones and maybe even put in a new Maxxair fan.
See my post, which also references JohnB's original post about replacing his defective vents with lots of pictures and a second post by JohnB for installing a Maxxair fan overtop one of the vents. These posts will give you all the info you need--use butyl putty tape and Dicor Self leveling caulk, not Eternabond. Several members have done vent replacements so ask away with any other questions.
Don't forget to put some boards and/or 1/2" plywood on the roof to make a work area.